Boron nitride (BN) is a compound of increasing importance for a variety of technological applications that take advantage of the beneficial properties of this material, such as its chemical inertness, high melting point, low density, high resistivity, high thermal conductivity and, in the case of the cubic form, an exceptionally high hardness. Additional applications, particularly in electronic devices, have been found for this material when applied as a thin film such as protective coatings, dielectric layers, diffusion sources, heat dissipation coatings, passivation layers, photoconductors, diffusion masks, wear resistant coatings, etc. Boron nitride films are deposited, with difficulty, on a variety of substrates by CVD, plasma enhanced CVD (PECVD), sputtering, electron beam evaporation, and ion beam deposition methods. Independent sources of a boron-containing species and a nitrogen-containing species are generally required for these methods.
Conventional methods of depositing films of BN require the use of flowing streams of a boron source such as boron trichloride or diborane and a nitrogen source such as ammonia into a hot reactor containing the substrate to be coated. The preparation of stoichiometric BN films requires close control of the ratio of reactant gases introduced into the deposition zone of the reactor. Complex metering equipment of expensive and/or hazardous materials are required. The quality of the resulting coatings is difficult to control and dependent on a highly accurate feeding of the appropriate proportions of reacting gases. These methods are generally not efficient, creating the need for trapping, disposal or recycling of unreacted feed material or gaseous by-products produced in the hot zone. Some methods require the use of microwaves to generate plasmas to enhance the rates of the reactions.
Two recent review articles, A. Lipp, et al., Journal of the European Ceramic Society, 5, (1989), 3-9, and S. P. S. Arya, et al., Thin Solid Films, 157, (1988), 267-282, describe preparation methods, properties, and applications of boron nitride films as well as a general description of the preparation, properties and applications of hexagonal boron nitride.